The invention relates to a hydraulic control system, in particular for activating a slewing gear of a digger.
A hydraulic control system is known from DE 196 20 664 C1. In the slewing gear control system disclosed by said publication, an adjusting apparatus is provided for adjusting an actuating piston, which is disposed between two actuating pressure chambers and influences the displacement volume of a hydraulic pump. The adjustment of the actuating piston is effected in dependence upon the pressure difference between two actuating pressure lines, which are connected each to one of the actuating pressure chambers. The actuating pressure in the actuating pressure lines is predetermined by two control lines connected to a manual control transmitter. Provided in each actuating pressure line is a separate braking valve, which throttles the return flow of hydraulic fluid from the actuating pressure chamber associated with the braking valve into a hydraulic fluid tank and hence enables the slewing gear to swing slowly outwards after the manual control transmitter has been returned into its neutral position by the operator. The effect achieved by the use of two separate braking valves, which are each connected to one of the working lines which connect a hydraulic motor driving the slewing gear to the hydraulic pump so as to form a working circuit, is that the slow braking by the braking valves is interrupted when the stewing gear swings out against a resistance, e.g. a heap of debris.
A drawback of the known hydraulic control system is however that the braking valves are disposed in the actuating pressure lines and are therefore biased by the actuating pressure. During the excursion of the actuating piston for accelerating the slewing gear, the hydraulic fluid filling the appropriate actuating chamber therefore flows through the braking valves, which are therefore exposed to increased fouling. The discharge of hydraulic fluid to the hydraulic fluid tank is effected by means of the manual control transmitter over relatively long line paths. Thus, the return flow of hydraulic fluid is restricted not only by the throttle provided in the braking valve but also by the area of the control lines and the opening area of the manual control transmitter. As a result, the time constant for the return flow of hydraulic fluid from the actuating pressure chambers of the adjusting apparatus may be adjusted reproducibly only to a limited extent by the throttle area of the braking valves. In said case, it has to be taken into account that the line length of the control lines, the manual control transmitter used and further structural parameters vary depending on the type of digger in which the hydraulic slewing gear control system is to be installed. The throttle area of the braking valves therefore has to be individually adapted to each type of digger, which entails a high assembly outlay. In addition, because the throttle areas of the braking valves used in DE 196 20 664 C1 are not adjustable, an adjustment after installation is not easily possible.
A further slewing gear control system is disclosed by DE 196 20 665 C1. In said slewing gear control system, the actuating pressure for the actuating pressure chambers of the adjusting apparatus is derived from the supply pressure of a supply apparatus via one or two pressure control valves. In said case, only one common braking valve for both actuating pressure chambers is provided, which is disposed in return flow direction downstream of a pilot device or pilot valve. In said refinement also, the return flow of hydraulic fluid first passes through the pilot valve, which likewise throttles the return flow, before reaching the braking valve. The effective throttle area therefore depends not only upon the throttle area of the braking valve but also upon the throttle area of the pilot valve as well as the areas of the connecting lines. The adjustment of the effective throttle area for the return flow of hydraulic fluid and hence the adjustment of the braking of the slewing gear is therefore made more difficult with said construction of the slewing gear control system also, especially as a variable, adjustable throttle area for the braking valve is not provided.
The object of the invention is therefore to indicate a hydraulic control system, in particular for activating the slewing gear of a digger, whereby the throttle area for the return flow of hydraulic fluid through the braking valves is definable more precisely and fouling of the braking valves is moreover counteracted.
The invention is based on the discovery that it is advantageous to dispose the braking valves, without interposing further valves, directly between the actuating pressure chambers of the adjusting apparatus and the hydraulic fluid tank. The resultant effect is short line paths for the return flow of hydraulic fluid from the actuating pressure chambers to the hydraulic fluid tank via the braking valve so that the effective throttle area depends substantially upon the throttle area defined by the braking valve and only to an insignificant extent upon the line areas. In the return flow path, apart from the braking valve, no further valves effecting an additional throttling are provided. By virtue of the fact that only the return flow of hydraulic fluid passes through the braking valves but not the flow of hydraulic fluid into the actuating pressure chambers in the event of acceleration of the slewing gear, the fouling of the braking valves is markedly reduced. In order, in the event of swinging-out of the hydraulic pump and loading of the actuating pressure lines with actuating pressure, to prevent a hydraulic short circuit of the actuating pressure lines via the braking valves towards the hydraulic fluid tank and, on the other hand, prevent a reflux of the returning hydraulic fluid into the actuating pressure lines or control lines, in each case a control valve is disposed in return flow direction downstream of a branch leading to the respective braking valve. According to the invention, the control valves and the braking valves are activated in such a way by the control pressure prevailing in the control lines that in the event of swinging-out of the hydraulic pump the control valves open and the braking valves close and, conversely, the control valves close and the braking valves open into their throttled valve position when the hydraulic fluid flows from the actuating pressure chambers back to the hydraulic fluid tank.
It is advantageous to provide the throttle area of the braking valves in an adjustable manner. This becomes possible only by virtue of the solution according to the invention, namely the arrangement of the braking valves not in the actuating pressure lines but in secondary lines, which branch off to the pressure medium tank, are biased with a lower pressure and exposed to less fouling. The braking valves in the known hydraulic control system take the form of seat valves so that they can withstand the actuating pressure there and be less susceptible to fouling. With seat valves, the construction of an adjustable throttle area is impossible or possible only with difficulty. An adjustable throttle area can be constructed more easily with a slide valve. A slide valve cannot however be used in the known hydraulic control system because, in the event of fouling, it can jam and hence lead to serious malfunctions. Given the development according to the invention, the use of a slide valve in the secondary line leading to the hydraulic fluid tank is however possible. In said case, the braking valve can comprise a braking valve piston, which is movable in a braking valve housing, cooperates with a control edge of the braking valve housing and has a bevel. The braking valve piston can strike against an adjustable stop which defines the throttle area of the braking valve, which throttle area is fixed by the overlap of the bevel of the braking valve piston with the control edge of the braking valve housing. In said case, the braking valve can comprise a braking valve spring which biases the braking valve piston towards the stop.
The control valve can take the form of seat valves and each comprise a control valve piston, which is movable in each case in a control valve housing. In said case, the control valve piston can have a conical portion, which cooperates with a valve seat so as to form a sealed seat. It is advantageous for the control valves to taken the form of seat valves because they then present a relatively high pressure resistance and insensitivity to fouling. Each control valve can comprise a control valve spring, which pressure the control valve piston against the valve seat. The control valve piston preferably takes the form of a stepped piston, wherein a step of the control valve piston is biased by the activating control pressure, thereby producing a hydraulically activated seat valve.
The braking valves and the control valves can be connected by a pressure change valve to the control lines. A supply device can be provided, which generates a supply pressure in a supply line. The actuating pressure can be connected in each case by an associated pressure control valve to the supply line, wherein the actuating pressure in the actuating pressure lines is adjusted by means of the control pressure prevailing in the control lines. When a pressure control valve spring is provided, which sets the actuating pressure slightly higher than the activating control pressure, then even given an imperceptible control pressure, there is a slight actuating pressure available, which is used to top up the actuating pressure chamber which increases in volume when the hydraulic pump swings back. A top-up device with a relatively large filter is therefore not required.
The control lines can be alternately loadable with control pressure by means of a control transmitter, which is connected to a control pressure supply and the hydraulic fluid tank.